1. Field of the Invention
The present invention generally relates to conductive pastes and multi-layer ceramic electronic components using the same. Specifically, the present invention relates to a conductive paste suitable for forming external electrodes of a multi-layered ceramic capacitor mainly composed of a Pb-perovskite-type ceramic component, and to a multi-layer ceramic capacitor using the conductive paste.
2. Description of the Related Art
A conventional multi-layer ceramic electronic component includes, for example, a ceramic composite, internal electrodes and external electrodes. A typical ceramic composite is made by baking a green composite comprising plural green ceramic layers composed of dielectric material. The internal electrodes disposed between the ceramic layers of the ceramic composite are formed by applying a conductive paste on each of the green ceramic layers by means of printing and then simultaneously baking the green ceramic layers and the applied paste. Each of the internal electrodes has one end exposed on a predetermined end face of the ceramic composite. The external electrodes are formed by baking a conductive paste applied on outer surfaces of the ceramic composite so as to provide the connection between the external electrodes and the exposed ends of the internal electrodes on the end face of the ceramic composite. The external electrodes are plated to improve the solder wettability required when the multi-layer ceramic electronic component is mounted on a circuit board or the like.
When silver (Ag) is employed as the conductive component of the conductive paste for forming the external electrodes, the Ag contained in the external electrodes diffuses into the electrically and mechanically connected internal electrodes during baking, thereby increasing the volume of the internal electrodes. Consequently, stress is generated in the vicinity of the end faces of the ceramic composite and is increased as the dispersion of silver progresses. As a result, the ceramic composite may suffer from cracks at an elevated baking temperature. Moreover, when a low-melting-point glass having a low flow temperature, i.e., a softening point of approximately 400xc2x0 C., is used as a glass frit in the conductive paste, the dispersion of Ag is accelerated by the glass, causing the ceramic composite to crack at a temperature lower than the marginal temperature at which the ceramic composite otherwise cracks as stated above. In this respect, when, for example, the ceramic composite is composed of a Pb-perovskite-type ceramic component such as PbTiO3 and the conductive component in the conductive paste for forming the external electrodes is Ag, the external electrodes are baked at a relatively low temperature ranging from 550xc2x0 C. to 650xc2x0 C. As the low-melting-point glass, Known is a glass mainly composed of B, Si and Pb, and containing Zn, Al and Ti in addition, as disclosed in Japanese Unexamined Patent Application Publication No. 7-161223.
However, the external electrodes baked at such a low temperature may not always be satisfactorily sintered. As a result, when the external electrodes are plated, the plating solution infiltrating into the external electrodes reaches the interface between the external electrodes and the ceramic composite, and, eventually, the interface between the internal electrodes and the ceramic composite, causing the internal electrodes to separate due to electrochemical reactions and the ceramic composite to crack. Such defects inside the composite often result in insufficient insulation and undesirable short circuits, thereby degrading the reliability of the multi-layer ceramic electronic component.
Moreover, insufficient sintering also results in the defective connections between the external electrodes and the internal electrodes, thus failing to achieve desired electrical characteristics as a multi-layer ceramic electronic component.
Accordingly, it is an object of the present invention to provide a conductive paste capable of inhibiting the generation of defects inside the ceramic composite and forming external electrodes having superior reliability as the electronic component while securing the connections between the internal and external electrodes, even when plating is performed on the external electrodes. Another object of the present invention is to provide a multi-layer ceramic electronic component having external electrodes formed by the conductive paste.
To these ends, a conductive paste according to one aspect of the present invention includes an Ag-based conductive component, an organic vehicle and a glass frit. The glass frit includes B in the form of B2O3, and Si in the form of SiO2, Pb in the form of PbO and Al in the form of Al2O3. The total content of SiO2 and Al2O3 in the glass frit is in the range of about 40 molar percent to about 56 molar percent.
A multi-layer ceramic electronic component according to another aspect of the present invention includes a ceramic composite having a plurality of stacked ceramic layers and external electrodes formed on outer surfaces of the ceramic composite. The external electrodes are formed by baking the conductive paste of the present invention described above.